Retinoic acid plays an important role in differentiation and development of a wide variety of fetal and adult tissues. Retinoic acid is produced from retinol in two steps: first, retinol is converted to retinaldehyde by a reversible oxidation, and then retinaldehyde is irreversibly oxidized to retinoic acid. The oxidation of retinol is the rate-limiting step in retinoic acid production that determines the overall rate of retinoic acid biosynthesis from retinol. During the previous funding period, we have identified two subfamilies of the human short-chain dehydrogenase/reductase (SDRs) superfamily of proteins that are active toward retinoids. When expressed in intact cells, RoDH-like SDRs confer the ability to oxidize retinol to retinaldehyde, whereas RalR1-like enzymes confer the ability to reduce retinaldehyde back to retinol. Based on these observations, we propose that both groups of human SDRs contribute to retinoid homeostasis in human tissues by regulating the equilibrium between retinol and retinaldehyde, and thereby, regulating the rate of retinoic acid biosynthesis. To test our hypothesis, we propose to characterize retinoid metabolism in human and animal tissues in the presence and in the absence of RoDH- and RalR1-like SDRs. Experiments under the first specific aim will test a hypothesis that silencing of RoDH-like SDR gene expression in human organotypic skin raft culture results in a decreased rate of retinol oxidation to retinaldehyde, whereas silencing of RalR1- like SDR gene expression results in a decreased rate of retinaldehyde conversion to retinol. Experiments under the second specific aim will complement the ex vivo studies in human skin rafts with in vivo studies in RalR1 knockout mouse model and will test a hypothesis that RalR1 is essential for the reduction of retinaldehyde to retinol in mouse tissues. Because in vitro the mouse ortholog of RalR1 is highly active toward medium-chain aldehydes in addition to retinaldehydes, we will also test whether RalR1 contributes to the reduction of medium-chain aldehydes. It has been reported that retinoid homeostasis is disrupted in cancer cells, in alcoholic liver disease and in fetal alcohol syndrome. The proposed studies will provide a better understanding of the molecular bases underlying disregulation of retinoid metabolism in various pathological states.